Atrial fibrillation (AF) is a serious disturbance in cardiac rhythm that increases the risk of stroke up to fivefold. Clinical guidelines therefore recommend anticoagulant medications for many patients with AF. Prior to 2010, warfarin was the only anticoagulant approved to prevent strokes in patients with AF. In 2010 and 2011, respectively, two new anticoagulants were approved for use with AF. While each of these drugs reduces the risk of stroke, they also pose significant risks -- including the risk of serious hemorrhage. Thus, clinicians must weigh the risks and benefits of anticoagulants in the face of competing risks. The decision to anticoagulate and the choice of anticoagulant in elderly adults is particularly complex, given the prevalence of multiple chronic conditions (MCC) in elderly populations. While stroke risk increases with age, the high prevalence of chronic conditions in the elderly can also increase the risks associated with anticoagulation. However, to date there is very little data to guide choice of anticoagulant in patients with AF and MCC, particularly given the availability of new anticoagulants since 2010. This study will evaluate the risks and benefits of anticoagulation by warfarin, dabigatran, or rivaroxaban in patients with AF who also have MCC. Specific aims are: Aim 1: Assess the prevalence and burden of MCC among elderly patients with AF. MCC prevalence and burden will be measured by chronic disease counts, polypharmacy, and functional limitations. Aim 2: Assess variations in warfarin, dabigatran, or rivaroxoban use for stroke prevention in patients with low, moderate, and high levels of MCC-related illness burden overall and by risk of stroke, bleeding, or death. Aim 3: Determine the relative likelihood of stroke, bleeding, or death for patients taking dabigatran, rivaroxaban, or warfarin, overall and within patient subsets defined by stroke risk, bleeding risk, and overall illness burden. The study will build on current work of the investigative team using previously acquired data to examine rates of stroke, hemorrhage, and death in elderly Medicare beneficiaries who receive dabigatran or warfarin for stroke prevention after initial AF diagnosis.